1. Field of the Invention
This invention relates generally to an electromagnetic detector designed to identify the position and orientation of a mobile body in a given space and more particularly to identify the exact position, in an aircraft, of the pilot's helmet, equipped with a helmet sight.
2. Discussion of the Background
An electromagnetic position and orientation detector generally includes a stationary transmitter with three coils that are excited alternately by a generator and a mobile sensor with three coils, each of which receives an induced signal relative to the excitation of each of the coils of the transmitter. A calculation means connected to the coils of the sensor make it possible to determine the position and orientation of the sensor as a function of the induced signals. In the case of identifying the position of the helmet in an aircraft, the cockpit of the aircraft constitutes the given space and the helmet constitutes the mobile body.
Such detectors are described, for example in French patent 2 458 838 and U.S. Pat. No. 4,396,885. To determine the position and orientation of a mobile body in a space, the transmitter occupies a stationary place in the space and the sensor is attached to the mobile body. At each position of the body, for each coil of the sensor, three signals occur which are induced respectively by the three coils of the transmitter. Thus, altogether nine induced signals are used to calculate the position and direction of the body.
This calculation is tainted with errors due to the imperfections of the channels that go from the electric excitation generator to the calculation means. Actually, these channels exhibit imperfections of gain both in transmission and in reception, including inaccuracies, non-linearities and drifts.
U.S. Pat. No. 4,054,881 discloses how to make an electromagnetic position and orientation detector with, in addition to a mobile sensor, a stationary antenna whose induced signal is used to produce a negative feedback in the transmission channels to keep the level of the electromagnetic field created by the transmitter constant. Such a device can eliminate the errors of the transmission channels, but has no effect on errors due to imperfections of gain which affect the elements located from (and including) the sensor up to (not including) the calculation means.